Rinse lamb roast, pat dry, and set on a metal rack in a shallow pan (at least 10 in. square). Mix ground cumin and 1/2 teaspoon each salt and pepper; rub onto roast, inside and out.
Bake lamb in a 450º regular or convection oven until a thermometer inserted horizontally through roast into center of thickest part reads 145º to 150º for rare, 35 to 40 minutes, or 155º for medium-rare, 40 to 45 minutes. If bone tips start to scorch, drape them with foil.
Meanwhile, in a 10- to 12-inch frying pan over medium-high heat, shake pine nuts frequently until lightly browned, about 3 minutes; pour into a small bowl.
To pan, add onion and sausages; stir frequently over high heat, breaking meat into small pieces, until lightly browned, about 10 minutes. Add broth and cover; when boiling, stir in peas and cover. When boiling again, stir in couscous, cover, and remove from heat. Let stand in a warm place 10 to 20 minutes.
As lamb roasts, in a food processor or with a knife, finely chop parsley, mint, and dill (or crumble dried herbs) and mix.
Transfer roast to a platter; keeping it warm, let rest 5 to 10 minutes. Stir herb mixture into hot couscous; fill center of roast with some of the couscous and spoon remainder around the meat. Sprinkle couscous with pine nuts. Cut lamb between ribs and serve chops with couscous. Add more salt and pepper to taste.